In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated from a camera or scanning device. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye receiver element in an image assembly. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta or yellow signals. The process is then repeated for the other colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen.
Thermal dye receiver elements used in thermal dye transfer generally include a support (transparent or reflective) bearing on one side thereof a dye image-receiving layer, and optionally additional layers, such as a compliant or cushioning layer between the support and the dye receiving layer.
The problem of controlling static charge is well known in the imaging industry. The accumulation of charge on film or paper surfaces leads to the attraction of dirt that can produce physical defects. It can also lead to transport issues during manufacturing, finishing and printing. The discharge of accumulated charge during manufacturing and finishing poses fire and safety hazards. The static problems have been aggravated by an increase in speed of manufacturing as well as printing.
In thermal transfer systems, a particularly unique source of charge generation is the dye transfer process itself wherein dissimilar materials are brought in close contact, followed by mass transfer (dyes and protective overcoat from donor to the receiver), further followed by rapid separation of the receiver and donor. The post-printing charge created in this manner is particularly undesirable for the receiver, since it can cause print-sticking, making it difficult for the user to subsequently handle prints, that is shuffle and stack prints, sort orders, separate prints, and other handling operations. For thermal dye receiver elements with a reflective support, typically comprising cellulosic paper, the post-printing charge created at the surface can induce “image charge” inside the paper (which is normally conductive) creating an electric field internal to the support that strongly holds the surface charge. This type of charge known as “polar” charge is very difficult to dissipate and can be a significant contributor to print-sticking problems.
Antistatic materials for imaging elements including thermal dye image receiver elements are known in the art, and include a broad variety of ionic and electronic conductive materials as well as charge dissipating surfactants.
U.S. Pat. No. 6,124,083 (Majumdar et al.) describes the use of sulfonated polyurethane film-forming binders and electronically-conductive polymers in various imaging elements including thermal imaging elements.
U.S. Pat. No. 5,710,096 (Ohnishi et al.) describes the use of an intermediate conductive layer in thermal transfer image-receiving sheets. These intermediate layers can include various conductive resins.
U.S. Pat. No. 5,384,304 (Kung et al.) describes the use of ionic conductors in a subbing layer under a solvent coated dye receiver layer for thermal receiver elements.
U.S. Patent Application Publication 2008/0220190 (Majumdar et al.) describes the use of aqueous subbing layers in extruded thermal dye receiver elements.
In general, electronic conductors have been found to be most effective in static dissipation but with higher cost. Ionic conductors, though inexpensive, show humidity dependent performance and may not be as effective in very dry conditions.
There remains a need to improve the conductive properties of the thermal dye image receiver elements so that static is properly and completely dissipated in a practical cost-effective way.